Detection and monitoring of partial discharge of a power line

ABSTRACT

There is provided a method that includes (a) acquiring a first spectral component of a single noise pulse on a power line, and a second spectral component of the single noise pulse, (b) determining that the single noise pulse is synchronous with a power voltage on the power line, (c) determining a first magnitude of the first spectral component, (d) determining a second magnitude of the second spectral component, and (e) determining a condition of the power line from the first and second magnitudes. There is also provided a system that employs the method.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/584,322, filed Oct. 20, 2006, which in turn claims priorityof U.S. Provisional Patent Application No. 60/819,072, filed Jul. 7,2006, the content of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to automated monitoring of the conditionof medium and high voltage cables and insulators in an electricallynoisy environment, and more particularly, to alternating current (AC)power line discharge. The present invention is particularly advantageousin a case where a power line communications infrastructure is availableto carry monitoring data to a central location.

2. Description of the Related Art

Partial discharge (PD) is a phenomenon that occurs in insulation thathas sustained damage, such as through aging, physical damage, orexposure to excessively high electric fields. PD may afflict cables,connectors, surge arrestors, and other high voltage devices. Faultyoverhead insulators may also generate noise with frequency and phasecharacteristics similar to PD. PD generates short pulses, whose durationis in the nano-second range or shorter. PD pulses tend to occur atcertain phases of an AC power voltage, and tend to be roughlysynchronized with the power frequency or twice the power frequency. PDis a member of a class of noise known as line-synchronized noise orline-triggered noise. PD pulses have a continuous broadband spectrumspanning at least a range between kilohertz and hundreds of megahertz.

Many techniques exist for sensing and identifying signals generated byPD on a power line, and for providing an indication of the location ofthe PD source. For example, Boggs, S. A., The Case for Frequency DomainPD Testing in the Context of Distribution Cable, IEEE ElectricalInsulation Magazine, Vol. 19, No. 4, July-August 2003, describes amethod for PD detection in the frequency domain, in which the frequencyaxis is synchronized to a phase of a power voltage on the power line.

These techniques are generally employed after a cable is suspected ofPD, and may not be practical for permanent deployment, due to lack of anability to readily communicating the information to some centrallocation or due to excessive cost. A disadvantage of some of thesetechniques is their requirement that PD signals be the strongest signalspresent, and so, such techniques may not function well in a fieldenvironment that includes strong radio signals that are picked up by thecable. The radio signals and other forms of external interference aretermed “ingress.”

SUMMARY OF THE INVENTION

There is provided a method that includes (a) acquiring a first spectralcomponent of a single noise pulse on a power line, and a second spectralcomponent of the single noise pulse, (b) determining that the singlenoise pulse is synchronous with a power voltage on the power line, (c)determining a first magnitude of the first spectral component, (d)determining a second magnitude of the second spectral component, and (e)determining a condition of the power line from the first and secondmagnitudes.

There is also provided a system that includes a coupler and a detector.The coupler couples a noise signal from a power line. The detector (a)receives the noise signal, (b) receives a signal having a fixed phaserelationship with a power frequency voltage on the power line, (c)acquires a first spectral component of a single noise pulse in the noisesignal, and a second spectral component of the single noise pulse, (d)determines that the single noise pulse is synchronous with the powerfrequency voltage, (e) determines a first magnitude of the firstspectral component, (f) determines a second magnitude of the secondspectral component, and (g) determines a condition of the power linefrom the first and second magnitudes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of a portion of a power distribution systemconfigured with an arrangement of components to detect PD on a cable inthe power distribution system.

FIG. 1B is another view of a portion of the system of FIG. 1A, showingan arrangement of a coupler on a cable.

FIGS. 2A and 2B are graphs that illustrate various waveforms in aprocess for detecting PD on a cable.

FIG. 2C is a table of values for a portion of the graphs of FIGS. 2A and2B.

FIG. 3A is a set of graphs that illustrate a use of a template, as analternative to a template discussed in the context of FIGS. 2A-2C.

FIG. 3B is graph of a template having a periodicity of 360 degrees.

FIG. 4 is a graph of a noisy spectrum and a product waveform.

FIGS. 4A and 4B are graphs of line-triggered noise power spectra, havingspectral peaks of different widths.

FIG. 4C is a graph of another line-triggered noise power spectrum.

FIG. 5 is a functional block diagram of a PD detector.

FIG. 6 is a functional block diagram of another PD detector.

FIG. 7 is an illustration of a portion of a power distribution systemthat includes a network of couplers and communications nodes deployed atmost or all distribution transformers in a neighborhood, configured todetect PD at a plurality of locations.

FIG. 8 is a graph of a line-triggered noise spectrum over a frequency of1 MHz-30 MHz

FIGS. 9A and 9B are block diagrams of a system for measuring PD over abroad frequency range.

FIG. 10 is a graph of several spectra acquired by the system of FIG. 9A.

DESCRIPTION OF THE INVENTION

In a power line communication system, power frequency is typically in arange of 50-60 Hertz (Hz) and a data communications signal frequency isgreater than about 1 MHz, and typically in a range of 1 MHz-50 MHz. Adata coupler for power line communications couples the datacommunications signal between a power line and a communication devicesuch as a modem.

An example of such a data coupler is an inductive coupler that includesa core, and a winding wound around a portion of the core. The core isfabricated of a magnetic material and includes an aperture. Theinductive coupler operates as a transformer, and is situated on a powerline such that the power line is routed through the aperture and servesas a primary winding of the transformer, and the winding of theinductive coupler serves as a secondary winding of the transformer. Thedata communications signal is coupled between the power line and thesecondary winding via the core. The secondary winding is coupled, inturn, to the communication device.

A further use for an inductive coupler is to place the inductive coupleraround a phase conductor, and sense high frequency energy generated byPD. The synergy achieved by a combination of functions, including acontinuous sensing of the cable and insulator condition, and datacommunications, is particularly advantageous.

Capacitive couplers may also be used for PD sensing and forcommunications. However, high voltage capacitors are themselvesvulnerable to the development of internal PD that may be difficult todistinguish from cable or insulation PD. Therefore, although capacitivecouplers may be used for sensing PD, inductive couplers are bettersuited for this task.

FIG. 1A is an illustration of a portion of a power distribution system100 configured with an arrangement of components to detect PD on a cablein system 100. System 100 includes a medium voltage underground cable,i.e., a cable 105, a distribution transformer 101, a ground rod 118, aninductive coupler, i.e., a coupler 120, and a PD detector 130.

Distribution transformer 101 is fed by cable 105, via an elbow connector107. Distribution transformer 101 has a neutral conductor 115 connectedto ground rod 118, and a secondary terminal 140. From secondary terminal140, distribution transformer 101 provides a low voltage at powerfrequency.

FIG. 1B is another view of a portion of system 100, showing anarrangement of coupler 120 on cable 105. Coupler 120 includes a magneticcore, i.e., a core 116, having an aperture 111 therethrough. Coupler 120operates as a current transformer, and is situated on cable 105 suchthat cable 105 is routed through aperture 111 and serves as a primarywinding of coupler 120. Coupler 120 also includes a secondary windinghaving leads 122 a and 122 b that run to PD detector 130 via a cable125. Cable 105 has concentric neutral conductors 110 that are gatheredtogether as a braid 112 and routed through aperture 111 to ground rod118.

The routing of braid 112 through aperture 111 results in cancellation ofneutral current induction into the coupler secondary, as described inU.S. Pat. No. 6,975,210. The net result is that coupler 120 sensescurrent in a phase conductor of cable 105, including power frequencycurrent and currents due to PD and ingress. The sensed current isavailable at the secondary winding, i.e., leads 122 a and 122 b, ofcoupler 120.

As an alternative arrangement of coupler 120 on cable 105, or in a casewhere cable 105 does not include concentric neutral conductors 110, suchas in a multi-phase power cable, coupler 120 may be placed directly oninsulation 106 of the phase wire. In such a case, coupler 120 wouldpreferably be packaged within a robust grounded conductive shieldcapable of routing fault current to ground, should the phase conductor'sinsulation fail.

Referring again to FIG. 1A, there is a fixed phase relationship betweenthe phase of a voltage (and current) on cable 105, and the phase of thelow voltage on secondary terminal 140.

PD detector 130 receives the sensed current from coupler 120 via cable125, and receives the low voltage at power frequency from secondaryterminal 140 via a cable 145. The low voltage at power frequencyprovides a phase reference for detector 130. PD detector 130 processesthe sensed current from coupler 120 to detect PD in cable 105, andprovides an output 135 that is connected to a communications link (notshown in FIG. 1A), allowing an ongoing stream of PD monitoring data toreach a remote monitoring station (not shown in FIG. 1A).

Coupler 120 also serves as a power line communications data coupler.That is, cable 125 is also routed to a communication device (not shownin FIG. 1A), and coupler 120 is employed to couple a data communicationsignal between cable 105 and the communication device.

A partial discharge generates a broad band of noise, and therefore, anindividual partial discharge includes spectral components throughout awide range of frequencies. Also, the duration of an individual partialdischarge is very brief, typically on the order of a few nanoseconds. Asa spectrum analyzer sweeps through a range of frequencies, the spectrumanalyzer acquires spectral activity that occurs at the point in timealong the horizontal scale to which the sweep has progressed. Thus,although the spectrum analyzer's horizontal scale is normally consideredas a frequency scale, it may also be interpreted as a phase scale and asa time scale.

FIGS. 2A and 2B are a set of graphs, namely graphs 210, 220, 230, 240and 250, that illustrate various waveforms in a process for detecting PDon cable 105. FIG. 2C is a table of values for a portion of graphs 220,230, 240 and 250.

Graph 210 is a normalized power line voltage wave, i.e., a cosinevoltage wave 213, of the voltage on cable 105. The horizontal scale ofgraph 210 is in units of phase. Positive peaks 211 and negative peaks212 alternate every 180 degrees.

Depending on the nature of the insulation damage of cable 105, PD pulseswill occur near positive peaks 211, near negative peaks 212, or nearboth of positive peaks 211 and negative peaks 212. Should PD occurmainly on one polarity only (i.e., only on positive peaks 211 or only onnegative peaks 212) the discharge will feature a periodicity of once percycle, or 360 degrees. Should substantial discharge occur on bothpositive peaks 211 and negative peaks 212, the discharge will feature aperiodicity of twice per cycle, or 180 degrees.

In cases where PD occurs on most power voltage cycles, and over a widerange of phase angles, it is advantageous to process the spectral dataacquired from single sweeps of a spectrum analyzer.

In other cases, PD spectral lines may appear sporadically, and be barelypresent or completely absent on some sweeps of the spectrum analyzer. Inthese cases it is advantageous to accumulate a set of the highest valuesof spectrum measured over a number N of frequency sweeps, and this isperformed by calculating a “max hold” value for each of the spectrallines.

A “max hold” of a spectrum is a plot of maximum magnitude values forfrequency components of the spectrum. That is, the max hold spectrum isa composite of a plurality of spectra, where the composite is configuredof a greatest magnitude detected for each of the plurality of spectra.For example, assume that a spectrum analyzer is evaluating a signal thatincludes a frequency component at 7.4 MHz. Further assume that thespectrum analyzer makes several sweeps, and that during the severalsweeps, the spectrum analyzer senses the 7.4 MHz component spanning arange of magnitudes from −45 dBm to −38 dBm. For the 7.4 MHz component,the spectrum analyzer would present a “max hold” value of −38 dBm.

Graph 220 is a synchronized “max hold” spectrum, i.e., a spectrum 227,of signals on cable 105, as indicated by a spectrum analyzer having itssweep triggered by a signal having a specific phase relationship withcosine voltage wave 213. More specifically, in graph 220, the spectrumanalyzer has its sweep triggered at a 0 degree phase angle of cosinevoltage wave 213, has a sweep duration of 1800 degrees, or 5 completecycles of cosine voltage wave 213, and has a start frequency of 5 MHzand a stop frequency of 12 MHz. Graph 220 has a logarithmic verticalscale 221, in dBm, and two horizontal axes, namely an axis thatdesignates a power line phase 222, and an axis that designates a noisefrequency 223.

In graph 220, since the spectrum analyzer is triggered at a 0 degreephase angle of cosine voltage wave 213, there is a fixed relationshipbetween the sweep of the spectrum analyzer and the phase of cosinevoltage wave 213. For example, when cosine voltage wave 213 is at phasesof 180 degrees and 360 degrees, the sweep of spectrum analyzer isevaluating frequencies of about 5.8 MHz and 6.5 MHz, respectively. Notethat a constant phase triggering of the spectrum analyzer sweep producesa fixed relationship between the power line phase 222 and noisefrequency 223. Thus, spectrum 227 is a line-triggered noise powerspectrum of signals on cable 105.

Spectrum 227 was experimentally measured at a PD magnitude of 25picocoulombs. When PD or other line-synchronized megahertz noise ispresent, a line-triggered noise power spectrum such as spectrum 227 willhave considerable periodicity, corresponding to the line frequency (360degrees), or twice the line frequency (180 degrees). Spectrum 227displays spectral components 226 peaking around each integer multiple of180 degrees of phase. Spectrum 227 also includes spectral peaks 224 and225 at approximately 6.9 MHz and 7.5 MHz, respectively, that do not fallwithin the spectrum at integer multiples of 180 degrees of phase.Spectral components 226 are indicative of PD. Spectral peaks 224 and 225are spectral contributions from a source other than PD, such as ingressfrom radio broadcasts or transient noise from switching loads on or offin a vicinity of cable 105.

An objective of the method being described herein is to recognize thatspectral components 226 are indicative of PD, and that spectral peaks224 and 225 are spectral contributions from a source other than PD.Accordingly, the method proceeds, as described below, to perform aspectral analysis of spectrum 227.

One possible technique for performing a spectral analysis of spectrum227 is to calculate a cepstrum of the signals on cable 105. A cepstrumis a Fourier transform of the logarithm of a spectrum. That is, it isthe result of taking the Fourier transform of the log-magnitude of thespectrum, as if the log-magnitude of the spectrum were a signal. Thus, acepstrum is a spectrum of a spectrum. In the context of the presentexample, the cepstrum of the signals on cable 105 would be found bycalculating a Fourier transform of spectrum 227. The cepstrum wouldreveal the intensified spectral activity, e.g., spectral components 226,at the frequencies that correspond to integer multiples of 180 degreesof phase of cosine voltage wave 213, thus revealing the existence of PD.

However, as mentioned above, for the detection of PD, the spectralregions of interest occur at integer multiples of 180 degrees of phase.Therefore, an alternative to calculating the cepstrum is to determine afundamental component of the cepstrum by correlating spectrum 227 with atemplate that screens the spectral regions of spectrum 227 at integermultiples of 180 degrees of phase. Graphs 230, 240 and 250 illustratethis technique.

For purposes of visualizing the correlation, zero-frequency componentsin spectrum 227 are eliminated, as explained below, by centeringspectrum 227 around an average value.

Graph 230 is a zero-centered version of graph 220, and thus shows azero-centered spectrum, i.e., a spectrum 232. Graph 230 effectivelyeliminates any zero-frequency components that may exist in spectrum 227,wherein spectrum 227 is viewed as a wave subject to a second process ofspectrum analysis. Spectrum 232 is obtained by computing an averagevalue of spectrum 227, and subtracting that average from every one ofits points, thus yielding spectrum 232. More specifically, for graph 220(and for convenience see FIG. 2C):

Average=((−48.0)+(−41.9)+ . . . +(−37.7)+(−38.9))/401=−52.3.

Points for graph 230 are obtained by adding 52.3 to the value for eachpoint of graph 220. For example, the first point of graph 220 has avalue of −48.0. Accordingly, the first point of graph 230 has a value of4.3, where:

4.3=(−48.0)+52.3.

Graph 240 is a template 242 for converting points of spectrum 232 (i.e.,graph 230) into another a set of points (discussed below in the contextof graph 250). Template 242 has values of either +1 or −1, and isconstructed symmetrical around zero. In template 242, the area abovezero is equal to the area below zero. Thus, template 242 has a net areaof zero. The values of +1 occur in vicinities corresponding to phasebeing an integer multiple of 180 degrees. The values of −1 occur wherethe value is not +1. For example, template 242 has a value of +1 in thevicinity of 180 degrees, and −1 in the vicinity of 270 degrees.

Graph 250 is a product waveform 252, obtained by multiplying each pointof spectrum 232 by a corresponding point of template 242. For example,as indicated in FIG. 2C, at an index k=1, spectrum 232 (i.e., graph 230)has a value of 4.3, and template 242 (i.e., graph 240) has a value of+1. Accordingly, product waveform 252 (i.e., graph 250) has a value of:

4.3=4.3×1.

Note that some points of product waveform 252 have negative values. Thisis due to an imperfect alignment of spectrum 232 with template 242. Thewidth of the peaks of spectrum 232 is not precisely 90 degrees, andtheir positioning is not precisely symmetrical around the multiples of180 degrees in graph 230.

For convenience hereinafter, we refer to a magnitude of a cepstralcomponent corresponding to a power line or doubled power line frequencyas a “PD score.” In the context of product waveform 252, the PD score isfound by summing the points of product waveform 252, and is equivalentto integrating a net area under product waveform 252.

${{PD} = {{\sum\limits_{k = 1}^{M}\; {{S_{k}(\phi)}{T_{k}(\phi)}}} = {\sum\limits_{k = 1}^{M}{P_{k}(\phi)}}}},$

where S_(k)(φ) is the k^(th) value of spectrum 232, T_(k) (φ) is thek^(th) value of template 242, and P_(k)(φ) is the k^(th) value ofproduct waveform 252. For product waveform 252:

PD = 4.3 + 10.5 + … + 14.6 + 13.4 = 2559.8.

The triggering source for the spectrum analyzer will generally not besynchronized with the center of the PD spectral peaks. Therefore, inpractice, the PD score is calculated a number of times, for a set oftemplates that have different initial phases, and thereafter, thehighest PD score is selected as an indicator of PD level.

FIG. 3A is a set of graphs, namely graphs 310 and 320, that illustrate ause of another template, as an alternative to template 242.

Graph 310 is a template 314 that, similarly to template 242, issymmetrical around zero, and has a net area of zero, but unlike template242 includes intervals of zero values, for example at point 312, and so,has narrower regions of +1 values, and narrower regions of −1 values.

Graph 320 is a product waveform 322 that results from template 310 beingapplied against spectrum 232. Graph 320, as compared to product waveform252, has zero values in phase regions that are not near integermultiples of 180 degrees.

Thus, template 314 minimizes the effects of alignment imperfection towhich template 242 is susceptible. Template 314 desensitizes the PDscore from variability of the width of spectral peaks 226, andincidentally, also causes the PD score to completely ignore any ingresscorresponding to frequencies where template 314 has a zero value.

FIG. 3B is graph of a template 332. Template 332 has a net area of zero,and a periodicity of 360 degrees. That is, template 332 has values of +1occurring in the vicinity of integer multiples of 360 degrees.

Referring back to FIG. 3A, note that template 314 is periodic, with aperiod corresponding to 180 degrees of a power voltage. That is,template 314 has a period that corresponds to one half of the period ofthe power voltage. In contrast, template 332 has a period thatcorresponds to the period of the power voltage. In a comparison oftemplate 314 and template 332, template 314 produces a large PD scorefor PD firing every half cycle of the power voltage, while template 332produces a large PD score for PD firing every full cycle.

In contrast with high PD scores calculated from processing spectra thathave clear PD, noisy spectra of similar peak magnitudes yield much lowerPD scores. This is because the spectral lines due to noise are randomrelative to power line phase.

FIG. 4 is a graph of a noisy spectrum 402 (designated by a fine line)and a product waveform 404 (designated by a heavy line). Noisy spectrum402 does not include PD. For example, noisy spectrum 402 could beproduced by applying a noise signal to cable 105, where the noise signalmagnitude is lower than a PD onset voltage. A spectrum analyzer displaysnoisy spectrum 402, after being centered on zero on the vertical scale.Product waveform 404 was produced by multiplying noisy spectrum 402 withtemplate 314, and yields a PD score of −57. The calculation of this PDscore is not shown herein, but it is obtained in a manner similar tothat shown for the calculation of the PD score for product waveform 252.

Recall that product waveform 252 yields a PD score of 2559.8, and thatproduct waveform 404 yields a PD score of −57. Thus, an environment inwhich PD is present (i.e., product waveform 252) yields a substantiallyhigher PD score than an environment in which PD is not present (i.e.,product waveform 404).

A further refinement may increase the certainty that a high PD score isdue to PD and not ingress. For this refinement, the PD score is measuredagain for a set of slightly different start and stop frequencies of thespectrum analyzer. If this measurement yields another high PD score, itreinforces a conclusion that there is PD or other line-synchronizednoise present on cable 105.

The width of the spectral lines represents additional information thatcan be derived from the acquired spectra. Some PD generators, especiallythose representing new PD sources, may have discharge occurring within anarrow range of phase angles, such as in a close vicinity of the peak ofthe power voltage. Other generators may have discharge over a broadrange of phases. Thus, the width of spectral peaks indicates a conditionof a power line.

FIGS. 4A and 4B are graphs of line-triggered noise power spectra havingdifferent widths of spectral peaks. FIG. 4A shows a spectral peak havinga width 420 of about 34 degrees, and FIG. 4B shows a spectral peakhaving a width 430 of about 133 degrees.

A template, and more specifically, a plurality of templates, can be usedto quantify the width of the spectral peaks. For example, in template314 (FIG. 3A), non-zero sections have a width 370. A plurality oftemplates similar to template 314 is created, where each of theplurality of templates has a different width 370. Thus, each of theplurality of templates has a different duty cycle. Each of the pluralityof temples is then used to produce a product waveform (similar to theproduction of product waveform 252) that is then used to produce a PDscore. The template whose duty cycle yields the highest PD score, isconsidered to have the width 370 that represents an approximation of thePD spectral line width.

Different physical mechanisms, or different PD sources, whose PDspectral components are generated during one polarity of the powervoltage, may differ in magnitude from the PD generated during the otherpolarity. This condition is evidenced by a different PD magnitude foreven multiples of 180 degrees than for odd multiples of 180 degrees.

For example, in FIG. 4A, the spectrum has a periodicity of about 360degrees, close to even multiples of 180 degrees (e.g., at 360 degreesand 720 degrees) relative to a first peak at about 45 degrees after atrigger phase. Such periodicity indicates that PD discharges occur onmostly one polarity of the power voltage. A more moderate degree ofdissimilarity between adjacent spectral peaks would be found in a caseof discharges occurring on both of the positive and negative polarity ofthe power voltage.

FIG. 4C is a graph of another line-triggered noise power spectrum, whichincludes peaks 440, 445 and 450. Peak 445 has a magnitude of about −80dBm, whilst neighboring peaks 440 and 450 have magnitudes of about −66dBm and −70 dBm, respectively, i.e., a difference of about 10 to 14 dBfrom peak 445. While periodicity may be apparent in FIG. 4C, it is lessapparent than in FIG. 4A.

A technique for quantifying the relationships between adjacent peaks isto first synthesize templates of 360 degree periodicity, whose initialphases are slightly varied from each other, and calculate PD scoresusing each template, until the phase is found that produces the highestPD score. This phase is noted as an optimum initial phase. Then, a newtemplate is synthesized with 360 degree periodicity, but with itsinitial phase shifted by 180 degrees from the previously detectedoptimum phase. Using the new template, a new PD score is calculated, anda deviation of a ratio of the two Scores from unity is termed “PDasymmetry,” a further useful parameter for quantifying PD. A ratio ofmagnitudes of alternating components indicates a condition of the powerline.

Further information may be gleaned from the PD signals, with regard tothe distance of the PD from the detector location. For example,underground cables tend to attenuate high frequency signals more thanlow frequency signals, so a downward trend of the frequency spectrum isan indication that the PD source may be distant from the sensinglocation.

Assume that the line-triggered noise spectrum is measured at aparticular point on a power line, and that the line-triggered noisespectrum has a low frequency (e.g., 5 MHz) spectral component and a highfrequency (e.g., 16 MHz) spectral component. If the magnitude of highfrequency component is approximately equal to the magnitude of the lowfrequency component, then the source of PD is likely to be near thepoint on the power line at which the spectrum is being measured. If themagnitude of high frequency component is less than the magnitude of thelow frequency component, then the source of PD is likely to be remotefrom the point on the power line at which the spectrum is beingmeasured. Moreover, given knowledge of the cable's attenuation of asignal as a function of frequency and cable length, the difference inmagnitude over frequency can be used to estimate a distance of the PDsource from the point on the power line at which the spectrum is beingmeasured.

PD detector 130 is contemplated as being able to perform any of thetechniques for detecting PD described herein. Nevertheless, below, thereis presented several exemplary embodiments of PD detector 130.

FIG. 5 is a functional block diagram of a PD detector 500. PD detector500 is an exemplary embodiment of PD detector 130, and includes ananalog amplifier 505, an attenuator 512, a spectrum analyzer 515, a maxhold calculator 520, a spectrum analyzer 525, a processor 530, acomparator 560, and a communications controller 535. PD detector 500receives a power line signal 502, e.g., from the secondary of coupler120 via cable 125 (see FIG. 1), and a low voltage at power frequency,i.e. a power frequency voltage 511, e.g., from secondary 140 via cable145 (see FIG. 1).

PD detector 500 determines a characteristic, e.g., a magnitude, of afundamental spectral component of a spectrum of a power spectrum ofnoise on a power line, and determines a condition of the power line,e.g., a presence of PD, based on the characteristic.

Analog amplifier 505 receives and amplifies power line signal 502, andoutputs an amplified analog signal 507.

Attenuator 512 receives power frequency voltage 511, attenuates powerfrequency voltage 511, and outputs a phase reference voltage 513.

Spectrum analyzer 515 receives phase reference voltage 513 and amplifiedanalog signal 507. Spectrum analyzer 515 uses phase reference voltage513 as a trigger, and so, is triggered at a constant phase of phasereference voltage 513. The phase of phase reference voltage 513 isessentially constant relative to the power voltage on cable 105. Thephase of PD pulses on cable 125 is closely related to the phase of thepower voltage on cable 105. Thus, phase reference voltage 513 is areference phase for analyzing PD. Hence, spectrum analyzer 515 acquiresa power spectrum of noise on cable 105 during a sweep of a range offrequencies that is triggered with respect to a phase of a power voltageon cable 105. Spectrum analyzer 515 outputs a logarithmic value of anamplitude of each spectral line, thus providing a line-synchronizedpower spectrum, i.e., a spectrum 517, of noise on cable 105.

Spectrum analyzer 515 can be implemented as a conventional spectrumanalyzer, or as a bandpass filter whose center frequency is sweptbetween a start frequency and a stop frequency, or as a superheterodynereceiver whose local oscillator frequency is swept between a startfrequency and a stop frequency.

Max hold calculator 520 receives spectrum 517. As mentioned above, PDspectral lines may appear sporadically, therefore, max hold calculator520 accumulates a set of the highest values of spectrum 517 measuredover one or more frequency sweeps, e.g., 1 to 7 sweeps, of spectrumanalyzer 515. Accordingly, max hold calculator 520 calculates a “maxhold” value for each of the spectral lines in spectrum 517, and yields amax hold spectrum, i.e., a spectrum 522. Thus, spectrum 522 is a maxhold version of the power spectrum of noise on cable 105, e.g., seespectrum 227 in FIG. 2A.

Spectrum analyzer 525 receives spectrum 522. When PD or otherline-synchronized megahertz noise is present, spectrum 522 will haveconsiderable periodicity, corresponding to the line frequency (360degrees) or twice the line frequency (180 degrees). To analyze thisperiodicity, spectrum analyzer 525 produces data representing a cepstrumof power line signal 502, i.e., cepstral data 527. Thus, cepstral data527 represents a spectrum of a power spectrum of noise on cable 105.

Processor 530 receives cepstral data 527, and ranks the magnitudes ofcepstral components. Processor 530 determines the phase 534 of astrongest fundamental spectral component 570 of cepstral data 527, andalso determines the fundamental spectral component (e.g. 360 degrees,180 degrees, or neither). If PD is present on cable 105, the fundamentalfrequency component will have a phase equal to either of (a) 360degrees, corresponding to a frequency of a power voltage on the powerline, or (b) 180 degrees, corresponding to twice the frequency of thepower voltage. The magnitude of the stronger of the two components isdesignated as the PD score. Processor 530 outputs a report 532 thatincludes the PD score, and the identity of phase 534, i.e. which of thetwo cepstral components, 180 degrees or 360 degrees, is present.

Comparator 560 receives report 532, which includes the PD score, andcompares the PD score to a threshold 555. Threshold 555 is a value setabove a level that represents background noise and ingress. If the PDscore is greater than threshold 555, then PD is present. If the PD scoreis not greater than threshold 555, then PD is not present. Comparator560 outputs a report 562 that includes either the PD score and theidentity of phase 534, or an indication that no PD is present.

Communications controller 535 receives report 562, and transmits areport 537 to a central monitoring station 540. Report 537 includeseither the PD score, or alternatively, an indication that no PD ispresent.

Central monitoring station 540 is represented as a box having a dashedline for a perimeter because central monitoring station 540 is not partof PD detector 500, but is instead, separate from PD detector 500.Central monitoring station 540 receives report 537 and maintains ahistory of PD scores from system 500. Central monitoring station 540also evaluates the PD scores over time, and if there is a change in thePD scores, or if a PD score exceeds a particular value, centralmonitoring station 650 will recommend corrective action.

FIG. 6 is a functional block diagram of a PD detector 600, which isanother exemplary embodiment of PD detector 130. PD detector 600,similarly to PD detector 500, includes an analog amplifier 505, anattenuator 512, a spectrum analyzer 515, and a max hold calculator 520,all of which function as described for PD detector 500. Additionally, PDdetector 600 includes an auto-centering module 605, vector multipliers610 and 615, integrators 620 and 625, a selector 630, a comparator 635,and a communications controller 640. As in PD detector 500, max holdcalculator 520 outputs a spectrum 522.

PD detector 600, similarly to PD detector 500, determines acharacteristic, e.g., a magnitude, of a fundamental spectral componentof a spectrum of a power spectrum of noise on a power line, anddetermines a condition of the power line, e.g., a presence of PD, basedon the characteristic. However, PD detector 600 does not obtain thefundamental spectral component in the same manner as PD detector 500.

Auto-centering module 605 receives spectrum 522, which is a max holdversion of the power spectrum of noise on cable 105, and zero centersspectrum 522 to yield a zero-centered spectrum 607, e.g., see spectrum232.

Vector multiplier 610 receives spectrum 607, and a template 606 having aperiodicity of 180 degrees (e.g., see template 314). Vector multiplier610 multiplies each point in spectrum 607 by a corresponding point intemplate 606. If template 606 is composed of values of only 0, +1 and−1, the multiplications performed by vector multiplier 610 either yielda product of 0, or are simply replicas or sign inversions of values inspectrum 607. Vector multiplier 610 outputs a product waveform 612.

Vector multiplier 615 receives spectrum 607, and a template 608 having aperiodicity of 360 degrees (e.g., see template 332). Vector multiplier615 multiplies each point in spectrum 607 by a corresponding point intemplate 608. If template 608 is composed of values of only 0, +1 and−1, the multiplications performed by vector multiplier 615 either yielda product of 0, or are simply replicas or sign inversions of values inspectrum 607. Vector multiplier 615 outputs a product waveform 617.

Integrator 620 receives product waveform 612, and integrates the areaunder product waveform 612. The integration can be obtained by summingthe points of product waveform 612. Integrator 620 outputs a candidatePD score 622.

Integrator 625 receives product waveform 617, and integrates the areaunder product waveform 617. The integration can be obtained by summingthe points of product waveform 617. Integrator 625 outputs a candidatePD score 627.

Selector 630 compares candidate PD score 622 to candidate PD score 627.As mentioned above, during the discussion of FIG. 3C, a template whoseperiod is 180 degrees produces a large PD score for PD firing every halfcycle of the power voltage, while a template whose period is 360 degreesproduces a large PD score for PD firing every full cycle. Accordingly,candidate PD score 622 will be greater than candidate PD score 627 forPD firing every half cycle of the power voltage, while candidate PDscore 627 will be greater than candidate PD score 622 for PD firingevery full cycle. Selector 630 selects the greater of candidate PD score622 and candidate PD score 627, and outputs the selected candidate PDscore as the PD score in a report 632.

Collectively, vector multipliers 610 and 615, integrators 620 and 625,and selector 630, in an arrangement 660 designated by a dashed line,determine a magnitude (represented by the PD score) of a fundamentalspectral component, e.g., 180 degrees or 360 degrees, of a spectrum ofthe power spectrum of noise on cable 105. That is, vector multipliers610 and 615 effectively serve to extract the fundamental spectralcomponent of a spectrum of spectrum 607, and integrators 620 and 627provide the magnitude value (represented by PD score). More generally,arrangement 660 determines a characteristic, e.g., magnitude, of thefundamental spectral component of a spectrum of a power spectrum ofnoise on a power line, e.g., cable 105.

The state of selector 630 represents an indication as to which of thetwo possible fundamental spectral components, 180 or 360 degrees, is thestrongest. This information is also included in report 632.

Comparator 635 receives report 632, which includes the PD score, andcompares the PD score to a threshold 637. Threshold 637 is a value setabove a level that represents background noise and ingress. If the PDscore is greater than threshold 637, then PD is present. If the PD scoreis not greater than threshold 637, then PD is not present. Comparator635 outputs a report 639 that includes either the PD score and theidentity of the strongest fundamental phase component, or an indicationthat no PD is present.

Communications controller 640 receives report 639, and transmits areport 642 to a central monitoring station 650. Report 642 includeseither the PD score and the phase of the strongest fundamental spectralcomponent (180 or 360 degrees), or alternatively, an indication that noPD is present. Central monitoring station 650 is represented as a boxhaving a dashed line for a perimeter because central monitoring station650 is not part of PD detector 600, but is instead, separate from PDdetector 600. Central monitoring station 650 receives report 642 andmaintains a history of PD scores 632 from system 600.

Central monitoring station 650 also evaluates PD scores 632 over time,and if there is a change in PD scores 632, or if PD score 632 exceeds aparticular value, central monitoring station 650 will recommendcorrective action.

In an alternative implementation of a PD detector, spectrum 517 istransmitted to a central location from equipment located at differentlocations, and all calculations and analyses are carried out at acentral processor. So, for example, with reference to FIG. 5, functionsperformed by max hold calculator 520, spectrum analyzer 525, processor530, integrator 550, and comparator 560 would be performed by thecentral processor. Similarly, with reference to FIG. 6, functionsperformed by auto-centering module 605, vector multipliers 610 and 615,integrators 620 and 625, selector 630, and comparator 635 would beperformed by the central processor.

FIG. 7 is an illustration of a portion of a power distribution system700 that includes a network of couplers configured to detect PD at aplurality of locations. System 700 includes distribution transformers703, 729 and 749, power cables 720, 740 and 755, couplers 702, 726, 732,746 and 752, and PD detectors 704, 727, 733, 747 and 753. Distributiontransformer 703, coupler 702 and PD detector 704 are arranged at alocation 705. Distribution transformer 729, couplers 726 and 732, and PDdetectors 727 and 733 are arranged at a location 730. Distributiontransformer 749, couplers 746 and 752 and PD detectors 747 and 753 arearranged at a location 750. Primaries of distribution transformer 703,729 and 749 are fed by cables 720, 740 and 755 arranged in a string,with power being supplied from cable 755.

Distribution transformer 729 receives power from power cable 740, andpasses power downstream via power cable 720. Each of couplers 726 and732 is connected to a single communications node (not shown) configuredas a repeater. Such a node may incorporate both of PD detectors 727 and733. PD detectors 727 and 733 each provide a PD score, with the higherPD score or other PD parameter indicating from which direction the PDnoise is arriving.

PD noise at location 715 originating in power cable 720 may propagateover power cables 720, 740 and 755, and may cause PD scores to rise atlocations 705, 730 and 750. A comparison of a relative increase in PDscores between outputs 710, 725, 735, 745 and 760 provides informationon the most likely general location of the PD source.

A monitoring station (not shown in FIG. 7) records a history of PDscores for multiple locations, and determines which cable or device isthe most likely damaged, based on an assumption that closer damage is toa coupler, the higher the PD level. The PD level also indicates theurgency of a site visit, for pre-emptive maintenance.

In system 700, since PD detectors 704, 727, 733, 747 and 753 are each ata different location, system 700 obtains an indication of a power linecondition detected at each of the plurality of locations. The PD scoresare communicated to the monitoring station, i.e., a central location,which compares the indications of the power line condition detected ateach of the plurality of locations to determine a most probable locationfor a source of partial discharge.

As explained above, the detection of PD involves spectral analysis of apower line signal across a frequency range, corresponding to a phaserange of 0 to N times 360 degrees. However, a sweep of the frequencyrange by a spectrum analyzer is relatively slow, and so, if the spectrumanalyzer acquires spectral components over a broad range of frequencies,the spectral components are likely to have been caused by a plurality ofdischarges. Consequently, a comparison of spectral components acquiredby a single spectrum analyzer implicitly assumes that all discharges areequivalent to each other. In practice, however, this equivalence is onlyapproximate, at best, and may not hold accurately for an entire sweep,much less a plurality of sweeps.

FIG. 8 is a graph of a line-triggered noise spectrum over a frequency of1 MHz-30 MHz, as acquired over a period of 1800 degrees of a powervoltage waveform. There is a spectral component 805 at about 5 MHz, anda spectral component 810 at about 16 MHz. Spectral component 805 wasacquired at a phase of about 250 degrees, and spectral component 810 wasacquired at a phase of about 970 degrees. That is, spectral component805 was acquired during a first period of the power voltage waveformafter a trigger, and spectral component 810 was acquired during a thirdperiod of the power voltage waveform after the trigger. Thus, thepartial discharge that generated spectral component 805 is not the samepartial discharge that generated spectral component 810. There is noguarantee that the characteristics of the partial discharge thatgenerated spectral component 805 are the same as the characteristics ofthe partial discharge that generated spectral component 810.Consequently, a comparison of the magnitudes of spectral components 805and 810 cannot be performed with a high level of confidence that thecomparison will yield a valid result.

Comparisons of PD detected at different locations will be more accurate,if each is based on the same set of discharges. Therefore, it isadvantageous to synchronize the triggers of the sweeps of all PDdetectors on a given feeder, and to accumulate the same number of sweepsat all detectors. When the detectors are part of a communicationsnetwork, such synchronization may be accomplished by the network.

As mentioned above, a partial discharge generates a broad band of noise,and therefore, an individual partial discharge includes spectralcomponents throughout a wide range of frequencies. For example, a singlepartial discharge would typically generate noise that includes aspectral component in the vicinity of 5 MHz, and simultaneously includesa spectral component in the vicinity of 16 MHz. Therefore, if twospectrum analyzers are employed such that one of the spectrum analyzersis sweeping in the range of 1 MHz, and simultaneously, the otherspectrum analyzer is sweeping in the range of 25 MHz, each of the twospectrum analyzers will capture a portion of the noise generated by thesame single partial discharge.

FIG. 9A is a block diagram of a system 900 for measuring PD over a broadfrequency range. System 900 includes a coupler 905, a low noisepreamplifier, e.g., an amplifier 920, five spectrum analyzers 925A-925E,five peak detectors and logarithmic converters 930A-930E, an analogmultiplexer 935, and an analog-to-digital converter (A/D) 945. System900 also includes a line frequency trigger circuit 960, and counter 965.

Coupler 905 is situated on a power line 910. A winding 915 from coupler905 is connected to amplifier 920.

Amplifier 920 receives, via winding 915, a signal derived from signalson power line 910. Amplifier 920 amplifies the signal from winding 915,and provides a signal 921 that includes a frequency component thatcorresponds to a frequency of the power voltage on power line 910, andalso includes noise that is propagating along power line 910. Signal 921is provided to each of spectrum analyzers 925A-925E, and to triggercircuit 960.

Trigger circuit 960 receives signal 921, and whereas signal 921 includesa frequency component that corresponds to a frequency of the powervoltage on power line 910, trigger circuit 960 provides a trigger 962that is synchronized to the power voltage on power line 910. Trigger 962is provided to each of spectrum analyzers 925A-925E, and to counter 965.

Counter 965 receives trigger 962, which resets and starts a count ofcounter 965. Counter 965 outputs a count 963, a count 970 and a count975. Count 963 is provided to each of spectrum analyzers 925A-925E.Count 970 is provided to analog multiplexer 935, and count 975 isprovided to a processor (not shown), as explained below.

Each of spectrum analyzers 925A-925E receives signal 921, trigger 962,and count 963. Each of spectrum analyzers 925A-925E are triggered bytrigger 962, and sweeps through a portion of a spectrum of signal 921.Count 963 controls the frequency sweep of each analyzer 925A-E, andcontrols the rate at which the sweeps progress. Thus, spectrum analyzers925A-925E each cover a different frequency range, but are synchronouswith one another, and sweep their respective ranges in parallel with oneanother.

For example, assume that we wish to analyze a spectrum of 1 MHz-30 MHz.Accordingly, spectrum analyzers 925A-925E sweep through frequencies asset forth in the following Table 1.

TABLE 1 Frequency Ranges of Sweeps of Spectrum Analyzers 925A-925ESpectrum Analyzer Frequency Range 925A   1 MHz-6.8 MHz 925B  6.8MHz-12.6 MHz 925C 12.6 MHz-18.6 MHz 925D 18.6 MHz-24.2 MHz 925E 24.2MHz-30 MHz  

Collectively, spectrum analyzers 925A-925E cover the full spectrum of 1MHz-30 MHz. The frequency bands swept by spectrum analyzers 925A-925Emay be arranged sequentially to cover a complete range of frequencies,as shown in Table 1, or may skip some frequency ranges that are not ofinterest or that have particularly high levels of ingress noise.

The duration of each sweep corresponds to one cycle, i.e., 360 degrees,of the power voltage on power line 910. Thus, for a 60 Hz power voltage,the duration of each sweep is 16.6 milliseconds. Each of spectrumanalyzers 925A-925E provides a spectral output.

Each discharge, in a cable suffering partial discharge, is extremelybrief, on the order of one nanosecond, and the existence of its spectralenergy is correspondingly brief. Spectrum analyzers 925A-925E incrementtheir respective frequencies stepwise, and dwell there for a relativelysubstantial period, e.g. 200 microseconds. A discharge may appear at anytime during this dwell time, and a peak detector is required to capturethe peak value of measurement, caused by this discharge.

Peak detectors and logarithmic converters 930A-930E receive the spectraloutputs of spectrum analyzers 925A-925E, respectively, and calculate alogarithm of the spectral outputs. Each of peak detectors andlogarithmic converters 930A-930E provides a logarithmic representationof the frequency swept by their respective spectrum analyzers 925A-925E.

Analog multiplexer 935, receives the outputs from peak detectors andlogarithmic converters 930A-930E, and also receives count 965. Based oncount 965, analog multiplexer 935 consecutively scans the outputs frompeak detectors and logarithmic converters 930A-930E, and provides amultiplexed output 940.

A/D 945 receives multiplexed output 940, and converts the multiplexedoutput to a data output 950. Data output 950 represents five spectra,each of which corresponds to 360 degrees of the power voltage on powerline 910.

Data output 950 is provided to a processor (not shown) that calculatesPD parameters. Count 975 is communicated to the processor, together withdata output 950, to identify from which analyzer and frequency rangedata output 950 originates. Count 975 is also indicative of the phase ofthe power voltage with which data output 950 is associated.

FIG. 9B is a block diagram of a system 901, which is another embodimentof a system for measuring PD over a broad frequency range. System 901 issimilar to system 900, however where system 900 uses spectrum analyzers925A-925E, system 901 uses a plurality of bandpass filters 985A-985E, toacquire discrete points of a power spectrum. Accordingly, in system 901,trigger 962 is provided only to counter 965, and counter 965 does notprovide count 963.

As with system 900, count 975 is communicated to a processor (notshown), together with data output 950. Count 975 provides phaseinformation so that points of data output 950 are recorded with respectto a phase of a power voltage on power line 910. Count 975 acts as alabel to identify which filter and frequency range was the source ofeach particular data output 950, and is also indicative of the phase ofthe power voltage with which each particular data output 950 isassociated.

Each bandpass filter 985A-985E is tuned to a different center frequencyand has a wide bandwidth (e.g. 1 MHz). One or more bandpass filters(e.g., 985A) have low center frequencies for which power line 910 doesnot appreciably attenuate PD, while other bandpass filters (e.g., 985E)have high center frequencies for which attenuation per unit distance issignificant. Frequency bands of bandpass filters 985A-985E arepreferably chosen to avoid frequencies of known sources of ingress, suchas broadcast stations.

Outputs 931A-931E of peak detectors and logarithmic converters 930A-930Erepresent an integration of the energy present in the filter passband.If little or no PD is present, outputs 931A-931E display a timevariation that is small and random relative to a power frequency. WhenPD is present, outputs 931A-931E will include a component that issynchronous with the power frequency or twice the power frequency.

Since each of bandpass filters 985A-985E is tuned to a different centerfrequency, their respective outputs are measurements of five separatespectral components. In the presence of PD, data output 950 includescomponents synchronous with a phase of a power voltage on power line910. Magnitudes of these components indicates a condition of a powerline.

FIG. 10 is a graph of the spectra acquired by system 900. FIG. 10includes five waveforms, i.e., one for each of the frequency rangesswept by spectrum analyzers 925A-925E. A waveform designated as “A”represents the frequency range swept by spectrum analyzer 925A, and awaveform designated as “C” represents the frequency range swept byspectrum analyzer 925C. Note that the horizontal axis represents phaseand runs from 0 to 360 degrees for each of the five spectra. At a phaseof about 250 degrees, waveform “A” includes a spectral component 1005 ata frequency of about 5 MHz, and waveform “C” includes a spectralcomponent 1010 at a frequency of about 16 MHz. Since spectral components1005 and 1010 both occurred at the same phase, they are both a result ofa particular partial discharge.

Whereas spectrum analyzers 925A-925E are triggered simultaneously, allfive spectra are derived from the same set of partial discharge pulses,and each partial discharge pulse is analyzed for its spectral strengthat five different frequencies. Thus, spectral components of a singlepartial discharge can be correlated with one another and compared to oneanother. For example, since spectral components 1005 and 1010 are causedby a single partial discharge, magnitudes of spectral components 1005and 1010 can be compared to one another, and a difference in themagnitudes can be attributed to attenuation of the partial discharge asit propagates along power line 910. Thus, system 900 is well-suited forevaluating a decrease of spectral magnitude with increasing frequency.

While various signal processing activities, (e.g., spectral analysis,peak detection, logarithmic scaling, determining parameters of powerline noise signals, determining whether PD exists, and determining PDstrength and location) are illustrated herein as being performed in a PDdetector located near a signal coupler placed on a power cable, itshould be understood that some or all of these signal processingactivities may be performed at a central location.

The techniques described herein are exemplary, and should not beconstrued as implying any particular limitation on the presentinvention. It should be understood that various alternatives,combinations and modifications could be devised by those skilled in theart. The present invention is intended to embrace all such alternatives,modifications and variances that fall within the scope of the appendedclaims.

1. A method comprising: acquiring a first spectral component of a singlenoise pulse on a power line, and a second spectral component of saidsingle noise pulse; determining that said single noise pulse issynchronous with a power voltage on said power line; determining a firstmagnitude of said first spectral component; determining a secondmagnitude of said second spectral component; and determining a conditionof said power line from said first and second magnitudes.
 2. The methodof claim 1, wherein said determining said condition comprisesdetermining a difference between said first and second magnitudes. 3.The method of claim 1, wherein said single noise pulse is a first singlenoise pulse, wherein said first and second spectral components aremembers of a first plurality of spectral components, wherein said methodfurther comprises: acquiring a second plurality of spectral componentsof a second single noise pulse on said power line, and determining, fromsaid second plurality of spectral components, that said second singlenoise pulse is also synchronized to said power voltage on said powerline.
 4. The method of claim 1, wherein said acquiring comprises routinga signal derived from said single noise pulse to a first filter and asecond filter having respective inputs in parallel with one another,wherein said first filter passes said first spectral component, andwherein said second filter passes said second spectral component.
 5. Asystem comprising: a coupler that couples a noise signal from a powerline; and a detector that: receives said noise signal; receives a signalhaving a fixed phase relationship with a power frequency voltage on saidpower line; acquires a first spectral component of a single noise pulsein said noise signal, and a second spectral component of said singlenoise pulse; determines that said single noise pulse is synchronous withsaid power frequency voltage; determines a first magnitude of said firstspectral component; determines a second magnitude of said secondspectral component; and determines a condition of said power line fromsaid first and second magnitudes.